Page display pointer: Considerably improved accessibility

- Updated pointer to move within content DOM so that you can back-focus
  into the pointer if desired.
- Added new "Section select mode" which toggles focusabiltiy for main
  content sections, with ability to show pointer via enter press on
  these.
- Updated pointer with proper input/button labelling.

Tested via orca screen reader on Firefox/Fedora/Gnome.
For #3975

tests/Entity/PageTest.php

@@ -50,6 +50,13 @@ class PageTest extends TestCase
         $resp->assertSeeText('Owned by ' . $owner->name);
     }
 
+    public function test_page_show_includes_pointer_section_select_mode_button()
+    {
+        $page = $this->entities->page();
+        $resp = $this->asEditor()->get($page->getUrl());
+        $this->withHtml($resp)->assertElementContains('.content-wrap button.screen-reader-only', 'Enter section select mode');
+    }
+
     public function test_page_creation_with_markdown_content()
     {
         $this->setSettings(['app-editor' => 'markdown']);